Transistor controlled power modules or "phase modules" are becoming increasingly important in the control of induction motors and other AC electrical devices. In a power circuit, it is necessary to create an AC output having an end use for running an induction motor. It is necessary to get the rotor to turn first. As a result, there must be an out-of-phase amperage and a power module forming a field used in a power motor.
One of the prior art problems in such circuits is maintaining a low inductance. When the powered circuit is operating in conjunction with a power module, the power switch allows a current to switch on and switch off very fast via the output of a power module. If there is a high inductance inside the module when the module switch is switched off, the amperage tries to fall very fast so that the slope of graph reflecting amperage versus time is very high. Because of this steep slope, there is a voltage overshoot, such as when trying to switch between the voltage and amperage. If the overshoot exceeds the overall breakdown voltage of the switch, then it is only possible to operate the switch at a fraction of the switch breakdown voltage. For example, if a field effect transistor (FET) used in switching applications can only handle 500 volts, and there is 150 volt overshoot, it is only possible to operate at the difference between those voltage levels, i.e., at only 350 volts.
Many of these power circuits use a power transistor circuit that is formed within a semiconductor substrate which, in turn, is enclosed within a power module housing, such as typically formed from an insulated material. These power transistor circuits could include field effect transistors or MOS controlled thyristors (MCT), and rectifiers. An example includes a circuit that forms a half-bridge circuit.
The power modules typically have positive and negative electrodes, which form anodes and cathodes that are connected to respective anode and cathode portions of silicon power transistor circuits formed in the substrate. These electrodes act as inductors and create voltage overshoot problems.
Snubber circuits (resistor, capacitor and diode) are also often used in conjunction with a power module and used for reducing the voltage overshoot by absorbing the energy. Typically, the snubber circuits are placed outside a power module. It is similar to adding an electrostatic discharge (ESD) circuit to absorb the energy. Snubber circuits control the voltage across the current through a controllable switch as the device is turning on or off. Typically, the snubber will limit the rate of rise in current as the device is turning on and limit the rate of rise in voltage as the device is turning off. However, snubber circuits can only limit the inductive overshoot by a limited amount. By adding a snubber package, the inductance stays the same. Only its effect is reduced because dv/dt at turn off is reduced.
It is desirable then to lower the inductance a greater amount in the power modules. However, there is only a limited amount of physical room in these power modules to implement mechanical and semiconductor design changes to reduce the inductance. By lowering the inductance in the power module, the voltage overshoot at turn off can be reduced which improves the safe operating area (SOA) curve. Another advantage in lowering the inductance in a power module is a reduction in the stray loss within the module due to current. Also, any reduction in inductance should preferably also reduce the direct cost of any materials and ease manufacturing.